By Kenneth W. Herrle, P.E., and Larry M. Bryant, Ph.D.
Explosive testing can reveal deficiencies in both new technologies and improperly designed window systems. Unfortunately, either through misinterpretation or ill-intent, compliance with selected test standards has been slowly falling by the wayside.
As terrorist bombings have become more frequent worldwide, mitigation techniques for such hazards have become commonplace in the design and construction of occupied facilities. Window design, in particular, has seen major changes in recent years largely due to the significant hazards posed by flying glass fragments. Historically, many building occupants who are seriously injured or killed in blast events received their injuries due to flying glass fragments from the building’s exterior window systems. This potential hazard has prompted many federal agencies to mandate explosive testing to pre-qualify window systems prior to being installed in their facilities.
Explosive testing can reveal deficiencies in both new technologies and improperly designed window systems, in addition to providing valuable data for improved analytical methods and software validation. Such tests can also provide:
- Proof of novel concepts
- General product validation
- Project-specific validation for a particular design loading and performance level
- Data for the development, improvement, and refinement of products, and analytical tools and methods
Because these tests provide real-world results, the explosive test bed has become a ‘proving ground’ for both newly-developed hazard mitigation technologies and project-specific designs. An explosive testing project typically begins by determining the test objectives, selecting the test method to be used, and determining the applicable test standard required for the project. Unfortunately, either through misinterpretation or ill-intent, compliance with selected test standards has been slowly falling by the wayside. Such practices can unknowingly compromise the validity of a test series. A basic level of knowledge regarding explosive testing and test standard provisions can help test sponsors ensure the efficiency and validity of their test series and keep them from falling victim to unscrupulous or unknowledgeable test providers. Manufacturers of blast mitigation window systems and products, who are typically the sponsor of many of these types of explosive tests, would achieve great benefit from knowledge of test procedures and test standard provisions. However, such knowledge can also prove useful for project managers, facility managers, designers, architects, and engineers who must make decisions regarding the use of tested systems in facilities with blast mitigation requirements.
Currently there are two independent test standards for explosive testing of window systems in the U.S. Both have their own specific markets:
- U.S. General Services Administration (GSA) Test Standard (GSA-TS01-2003)
- Used by the GSA and other non-Department of Defense (DoD) agencies who comply with the ISC Security Design Criteria
- Available at no cost from the GSA (http://www.oca.gsa.gov/ )
- ASTM Test Standard (ASTM 1642-04)
- Widely used by some DoD components
- Available for purchase from ASTM (http://www.astm.org/ )
At this time, universally accepted blast test standards in the U.S. only exist for window systems. Accepted blast test standards have yet to be developed for other façade elements such as doors and walls.
Test Standard Overview and Compliance – The Double Standard
Both the GSA and ASTM test standards were developed to ensure an adequate measure of standardization and quality assurance in the testing of window systems. Each standard contains the following general provisions:
- A listing of terms and definitions associated with the explosive testing of window systems
- Performance Criteria/Hazard Ratings
- A listing of test requirements, which include:
- Allowable test methods (shock tube or high-energy explosive arena testing)
- Blast load and explosive charge requirements
- Test site and reaction structure requirements
- Data collection and documentation requirements
- Test specimen requirements
- Test report requirements
With many similarities between the two, testing to either standard can typically be accommodated at the same test site through modifications of the test setup. Unfortunately, with such similarities, distinctions between these two test standards have become increasingly blurred as test providers have slowly been intermixing differing components of each standard. In addition, failure to include mandatory test requirements has become more commonplace. Such behavior violates the protocol outlined in either test standard and has the potential for invalidating test results for unsuspecting test sponsors.
Fortunately, test sponsors can take precautions during pre-test planning to help protect the integrity of their test series. The following recommendations can prove helpful in doing so.
For starters, the test sponsor should obtain a copy of the selected test standard prior to testing and become familiar with its requirements. As part of the familiarization, distinction should be made between "test options" and "test requirements." Test requirements are typically accentuated in the standards by phrasing such as “must,” “shall” and “as a minimum.” Each of these items must be accounted for in the tests, whereas test options do not. For an explosive test to meet a selected test standard, all test requirements specified in the standard must be met.
The test sponsor should ensure that the test method being used meets the objectives of the test. For example, if the objective is to verify that the tested windows meet a specific GSA Performance Condition, it is unacceptable to perform the test using the ASTM Test Method. Conversely, if the test windows must meet a specific ASTM Hazard Rating, testing performed using the GSA Test Method is unacceptable. In short, differing test methods and performance ratings cannot be intermixed; they are exclusive of one another.
Although explosive tests can be configured to meet both the ASTM and GSA test standards, there are numerous differences between these two methods that must first be addressed. Several differences in test requirements between these two standards include:
- Interior high-speed photography requirements: Although the ASTM Test Method allows the use of high-speed video, it is not necessary for meeting the test method requirements. The GSA Test Method, on the other hand, requires that, as a minimum, interior high-speed video be recorded for each test specimen.
- Pressure gauge requirements (both number and location): The ASTM Test Method requires the use of three reflected pressure gauges mounted on the exterior of each test structure and use of a free-field exterior pressure gauge located near one of the test structures. The GSA Test Method, on the other hand, requires the use of two reflected pressure gauges mounted on the exterior of each test structure and the use of at least one interior pressure gauge located inside of each test structure (or inside of each partitioned volume).
- Requirements specifying the minimum number of test samples: The ASTM Test Method requires testing of a minimum of three samples for each blast load environment plus disassembly and measurement of a fourth sample. The GSA Test Method does not specify a minimum number of test samples (one is typically used), or that sample disassembly is required.
While these are just a few of the differences between the two test standards, they are provided as a starting point for the test sponsor to assure that the test provider is conducting the test in compliance with the correct test standard. The test standard selected for the project should be consulted for the full listing of requirements that must be met.
Prior to testing, the test sponsor should make an inquiry with the test provider regarding the adequacy of the test structures planned for the test. If the test structures do not meet the requirements, the validity of the test results may be questioned. The test sponsor should assure that all test structures are fully enclosed (on all sides) to prevent infill loads, and that the response limits of the test structures will be in adherence with the selected test standard when exposed to the target blast loads. Questions regarding acceptable test structure response limits should be raised if the test structures are constructed of materials such as wood or lightweight metal sheeting, as these materials are not commonly used in blast-resistant construction.
If multiple window units are tested side-by-side in the same test structure, make sure that provisions have been made to distinguish between the glass fragments generated by each window unit. This can be accomplished by using a different color glass for adjacent window units, or by physically segmenting the test structure into individual compartments to avoid fragment intermixing. Also be on the lookout for items that may affect the response of the window systems such as test structure framing components located directly behind window mullions or items located directly behind the window that may obstruct fragment flight.
An additional item that may compromise the validity of an explosive test, and should be addressed during pre-test preparations, is the failure to meet the minimum blast load requirements specified for the test. Typically, window systems are tested to meet a specific set of criteria which consist of load requirements for both pressure (psi) and impulse (psi-msec). These load requirements are minimum baseline values for meeting the criteria. Post-test reporting of sub-baseline test loads as having met the baseline requirements, based solely on the rationale that the loads are within a specific percentage of tolerance, is unacceptable. Tests that do not meet or exceed minimum baseline values (both pressure and impulse) have not met the specified test criteria, and the validity of the test results in such cases is often challenged. Prior to testing, the test sponsor should verify with the test provider that the target test loads are expected to be either met or exceeded, and the test sponsor should request proof that similar loads have been achieved on previous tests. Standard blast load calculations assume that the explosive event takes place at sea level and that the reflecting surface of the target structure is infinitely large. When calculating charge weight and standoff combinations for explosive tests, adjustments must be made for both the altitude of the test site (if not at sea level) and clearing effects on smaller test structures. Failure to do so will result in blast loads that are lower than predicted.
Both test standards provide a list of required items that must be included in the test report as a minimum. The test sponsor should verify with the test provider that all test report requirements will be met. Omission of any of the required items is in violation of the test protocol and compromises the integrity of the report.
Prior to arranging for explosive testing, the final test objective must be decided. Generally, there are two major categories of test objectives:
- Proof testing
- Data collection
Although these objectives have differing ultimate goals, they share many common benefits. For example, although the ultimate goal of a proof test is to obtain a “pass/fail” rating for a given specimen and load combination, data collected from the test is also useful for other purposes, such as development and validation of analytical methods and software, as well as improvement of a tested product. Such additional benefits may include:
- High-speed video footage for analyzing the time-dependent response of window systems
- Pre-test and post-test photographs and measurements for analysis/comparison of response between tested window systems, including failure modes and details
For example, high-speed video of the specimens during a test has been very useful in determining actual sequence and modes of failure in complex systems. Such documentation has also allowed development and validation of analytical models. For example, the bite response model currently implemented in the national standard WINGARD series of window blast analysis programs evolved from test observations.
Similarly, testing conducted for the primary purpose of collecting data may also provide a “pass/fail” rating as a byproduct of the test.
Explosive testing is typically performed in either shock tubes or in large-scale, open-air arenas. Shock tube testing is generally less expensive but is not as realistic as open-air tests.
Shock Tube Testing. During shock tube testing, the test specimen is mounted at the end of a structural tube. A pressure pulse, generated at the opposite end of the tube from the test specimen, travels down the tube and impacts the test specimen. Pros and cons of shock tube testing include:
- Less expensive than large-scale arena testing when testing only a few samples
- Readily reproducible loads
- Can be conducted quickly
- Generally limited to one specimen per test
- Generally limited to relatively small test specimens
- Difficult or impossible to achieve realistic open-air blast waveforms (typically the positive phase impulse may be too high and negative phase effects cannot be consistently replicated)
- Difficult to obtain high-quality video footage of the response
In general, shock tube testing can be a useful tool for conducting expedient, inexpensive snapshots of the hazard mitigation potential for small specimens. However, a full-scale specimen will most likely have to undergo open-air testing to receive true validation within the government and commercial markets.
Large-Scale Arena Testing
Large-scale arena testing is conducted on an outdoor test bed using actual high-energy explosives and full-scale test specimens. The pros and cons of large-scale arena testing include:
- Large size (often full-scale) specimens may be evaluated.
- Multiple specimens may be evaluated simultaneously, reducing overall cost per specimen.
- Replicates actual explosive environments, resulting in realistic blast loading that includes both positive and negative phase.
- High quality video footage of the response is typically obtained, providing good scientific data, as well as dramatic marketing/demonstration video.
- More expensive than shock tube testing when testing only one or a few samples.
- Adjustments to the test structure and/or charge size may be required to avoid reduced impulse due to clearing effects when using smaller test structures.
- May require longer lead times in test planning.
- Some variability in results due to real-world environment.
In general, large-scale arena testing allows documentation of a real-world response of multiple, full-scale test samples in an actual, high-energy explosive environment and generally provides more validity in both the government and commercial markets than shock tube testing.
Both the ASTM and GSA test standards allow the use of either shock tube or large-scale arena testing.
Several additional suggestions that are not in either the ASTM or GSA test standard, but should be used by the test sponsor to help insure a successful test series include:
- Performing a pre and post-test walkthrough/inspection of the test setup with the test sponsor. During this inspection, be sure to check for correct installation of the test windows and be on the lookout for anything that may be out of place or doesn’t look correct. Be sure to alert the test sponsor of any concerns.
- Being prepared for the unexpected. Anomalies sometimes happen in explosive testing, so it’s best to know this before hand. In addition, many test sponsors arrive at the test bed with preconceived expectations of what should happen and how their product should perform. Please be aware that the purpose of the explosive test is to determine the real-world response of the window system (what really happens). Some test items perform better than expected, some perform worse, and some perform just as expected.
- Taking time to make sure that everything in the test setup is correct before proceeding with the test. Explosive testing is very expensive and there is only one chance to get it right. Do not rush.
- Shipping extra test samples to the test site. Window samples are sometimes damaged during shipment. In tests using multiple window systems, shortage of one window equates to loss of a tested system and a resulting higher per-unit test cost.
- Being sure to ask plenty of questions and expect the test provider to provide sufficient answers.
Additional measures can be taken to ensure compliance with test standards, but test sponsors must be aware that due to the complex nature of explosive testing, data, high-speed photography, or other critical test items may not perform as intended due to unforeseen circumstances. Test providers should utilize a high level of quality control to mitigate the risk, but there is no guarantee that the testing will always perform as desired.
Although explosive testing of window systems can provide a significant contribution toward protecting the occupants of constructed facilities, caution must be exercised to assure that all goals and requirements of the testing are met. With careful planning, well-informed decisions can be made which should lead to an efficient, productive, and valid explosive test series.